Are we looking at four dots flashing? Or are we seeing two dots moving around? To me, it looks like two dots bouncing up and down like rubber balls, but you might see something completely different.

Do you see two dots moving from side to side? Perhaps you see them moving around clockwise — or counter-clockwise. Or maybe you see four dots. In any case, the caption tells us that the lights are flashing at a frequency of five Hertz.

Further down the thread, d023n played with this GIF so viewers could analyze it at different speeds.

Here's one at two Hertz, where you can see all the movement possibilities if you just stare at it for awhile.

Here's a sped-up version at 10 Hertz, which is way too frantic to wrap your brain around.

Now, the fun of discovery: Have you ever seen train wheels (or anything else spinning) that suddenly start to look like they're standing still? Or even spinning backward, although you can see the train is still moving forward?

Just like your TV or computer screen, human eyes have a "frame rate." It's called the critical flicker frequency, and it's what determines the frequency at which we need to see images for them to look smooth instead of flickering. The strobe effect can mess with that flicker frequency, resulting in the trick of the eye.

It's actually four dots blinking, but the speed at which they blink determines what your brain thinks it sees.

The blinking speed of these lights determines your perception of motion. The longer you stare at it, the more you might start to see different behaviors.